Soggy Spot and high water bill, but can't find the leak even after shutting the backflow valve and isolation valve.

Hey all, I've got a bit of a mystery on my hands. Noticed a soggy area in my backyard and a much higher-than-usual water bill. Dug around near a sprinkler head where the ground is wet, expecting to find a broken irrigation pipe — but no visible leak. Here's what I've done: Turned off the backflow preventer and isolation valves for the sprinkler system (this stops the sprinklers from running). Even with those off, the soil around that area stays wet and seems to slowly fill back up. There are no other known water lines or connections in that part of the yard. So if the irrigation is off, where could the water still be coming from?